1. Field of the Invention
This invention relates to solid rocket motor nozzle ablative composites. More specifically, the present invention relates to phenolic ablative composites which are self venting.
2. Technology Review
The combustion of a propellant in a rocket motor creates a hostile environment characterized by extremely high temperature, pressure, and turbulence. The combustion temperature within the motor typically exceeds 6,000.degree. F. Pressure within the motor typically exceeds 1,000 psi. Gas velocities typically range from Mach 0.02 in the inlet region to Mach 10+at the aft end of the rocket motor nozzle. This environment is particularly hostile in a solid rocket motor because its combustion gas typically contains chemical species and particulates which tend to physically and chemically erode exposed rocket motor nozzle components. While the combustion of the rocket propellant is usually brief, the conditions described above can destroy insufficiently protected or inferior rocket motor nozzles prematurely and jeopardize the mission of the motor.
Parts of a rocket nozzle which are exposed to the high temperatures, pressures, and erosive flow conditions generated by the burning propellant must be protected by a layer of insulation. Various materials have been tried as insulation, such as silica dioxide, glass, or carbon fiber reinforced silicone and/or polyisoprene elastomers, but reinforced resin composite materials are most commonly used. The reinforced resin composite materials are typically prepared by taking squares or plies of resin impregnated carbon or graphite cloth and positioning them in the desired shape. Phenolic resins, such as phenol-formaldehyde resin, are particularly preferred because of their heat resistance, good insulation properties, low cost, and ease of handling and manufacturing.
Solid rocket motor nozzle ablative components consisting of chopped squares of carbon or graphite cloth phenolics molded at low angles (i.e., &lt;40.degree.-60.degree.) to the gas flow, exhibit localized increased erosion, called spalling, resulting in higher than expected erosion. This erosion or spalling may be attributable to the molded part's inability to vent pyrolysis gases that form a small distance below the flame surface of the component.
These molded components are prepared by "chopping" carbon or graphite cloth impregnated with phenolic resin into small squares, often from 0.375 inch to 1.0 inch squares. These chopped squares are then placed in a suitable mold and cured under heat and pressure. Chopped squares molded components represent cost savings in that they do not require slitting and stitching operations required of tape wrap materials, and almost 100% of the prepreg broadgoods can be utilized.
Previous designs incorporating hundreds or thousands of holes drilled to given depths, in a geometric grid, successfully minimized the spalling effect described above. However, this operation was very time consuming and expensive.
Solid rocket motor ablative components are also prepared by tape wrapping carbon or graphite cloth impregnated with phenolic resin around a mandrel or mold. Low tape wrap angles, i.e., &lt;30.degree. (the tape ply angle to gas flow surface), are desirable because of less distortion of the tape required to wrap the component. However, low angle tape wrapped components sometimes exhibit "ply lifting." This phenomenon appears as localized separations between adjacent plies and the inside (gas flow surface) ply ends tend to bend inward. Separations have also been observed below the surface between plies in the phenolic resin char or decomposition transition temperature zone.
It will be appreciated that there is a need in the art for carbon or graphite ablative compositions which avoid spalling and high erosion rates associated with low cost carbon or graphite phenolic molded parts and which also avoid unpredictable ply lifting with low angle-to-centerline tape wrapped carbon or graphite composite ablatives.
Such carbon or graphite ablative compositions are disclosed and claimed herein.